Acoustic flow meters are becoming increasingly popular in measurement of fluid flow in pipes. Good accuracy can be achieved with essentially no pressure drop due to metering; thus, pumping energy is conserved. Moreover, such flow meters can be mounted on the exteriors of pipes (albeit with some loss in accuracy) and, thus, they are desirable for use in measuring flow of hazardous materials. Further, acoustic flow meters do not increase markedly in cost with the size of the pipe being monitored. Among such flow meters are those described in U.S. Pat. Nos. 3,940,985; 3,564,912; 3,537,309; 3,697,936; 3,720,105; 3,881,352; 3,882,722; 3,886,794; 3,901,078; 2,914,998; 3,974,693; 4,024,760; 3,546,935; 3,564,912; and 3,918,304.
Nevertheless, it remains a problem to embody an improved combination of high accuracy and low cost in an acoustic flow meter. The incremental value of even a small increase in accuracy can be substantial in applications on large pipelines, e.g. large oil pipelines, petroleum refinery lines, municipal water service lines and the like. Also, such meters are of particular value in testing and calibration measurements in large hydraulic installations, e.g. in testing of large hydraulic machinery and in power plant flow installations. In some of the latter installations the pipe diameter may be 30 feet or more.